Unpressurized heat accumulator with compensation line

ABSTRACT

A heat accumulator for storing and providing heat energy accruing when power is generated, includes a tank for a fluid, including an expansion line, which exits from the tank and which extends above the tank in order to increase the static pressure in the tank, the expansion line having an open end opposite the tank-side end for establishing a connection between the tank interior and the atmosphere, wherein the expansion line extends into the tank. A method for storing and providing heat energy accruing when power is generated wherein the fluid is used in a district heating grid and is stored in a tank.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Stage of International Application No. PCT/EP2014/075808 filed Nov. 27, 2014, and claims the benefit thereof. The International Application claims the benefit of European Application No. EP13195629 filed Dec. 4, 2013. All of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a heat accumulator, in particular a heat accumulator for storing and providing heat energy, which occurs during the generation of electricity, by means of a fluid and concerns the unpressurized storage at temperatures above 100° C. The invention moreover relates to a combined heat-and-power plant, and to a method for storing and providing heat energy, which occurs during the generation of electricity, by means of a fluid.

BACKGROUND OF INVENTION

The high degree of fluctuation in the feeding of renewable energy (sun/wind) into power grids causes a dilemma for combined heat-and-power (CHP) plants in that they need, on the one hand, to deliver thermal energy according to district heat requirements and, on the other hand, to deliver electrical energy according to grid requirements, wherein it can then be uneconomical, for example at night or weekends, to burn fossil fuels in order to generate district heat because the price of electricity is then often very low (and may in future even get lower and lower). District heat accumulators, preferably in the form of stratified hot-water accumulators, are therefore increasingly used in order to decouple the production of thermal and electrical energy. Because unpressurized stratified accumulators can only be charged with water at temperatures of <100° C. (in practice this is usually limited to <95° C.) and because heat exchangers with an additional terminal temperature difference often need to be interposed between the accumulator and the district heat grid (because the district heat grid often has a considerably higher operating pressure than the buffer accumulator), the temperature that can actually be used falls in part in discharge mode to below 85° C. In transitional periods (spring/fall) and especially in winter, however, the conventional district heat grids are operated at supply temperatures significantly above 95° C. (up to approximately 135/140° C.): except in summer at district heat supply temperatures below 85-95° C., it is therefore impossible with the concept currently in common use to serve the district heat grid only from the heat accumulator alone and to completely switch off the CHP power plant or the fired or electric auxiliary boiler.

Hot water containers or accumulators are known, for example, from GB 2 195 172 A and WO 2009/135310 A1 or DE 43 05 867 A1, in which an unpressurized hot water accumulator for district heating systems is disclosed.

In order to be able to charge stratified hot-water accumulators with higher district heat supply temperatures above 95° C., pressurized containers are used in part which can be charged and discharged, owing to the pressure-dependent saturation temperatures, with correspondingly higher supply temperatures. The big disadvantage of pressurized containers, however, is a significantly higher testing cost during production and recurring tests (for example, pressure tests with cold water after 10 years, recurring visual testing of all welds: the whole insulation of the tank would need to be removed to do this). In addition, the insurance premiums for pressurized containers increase sharply depending on the pressure and the volume (because the risk is estimated by insurers as very high) and hence it is currently very rare to use pressurized containers for large accumulator volumes for cost reasons.

SUMMARY OF INVENTION

An object of the invention is to provide a method for storing and providing heat energy, which occurs during the generation of electricity, by means of a fluid.

In order to carry out the method, a heat accumulator for storing and providing heat energy which occurs during the generation of electricity is required, comprising a tank for a fluid, providing a compensation line, which starts from the tank and extends above the tank in order to increase the static pressure in the latter, with an open end, opposite the tank-side end, for establishing a connection between the tank interior and the atmosphere, wherein the compensation line extends into the tank.

The invention makes use of the fact that tanks (of whatever height) that are open to the atmosphere are considered not to be pressurized containers but buildings or equipment, regardless of the static pressure and the operating pressure, because pressure is defined as elevated pressure relative to the atmospheric pressure. It is not possible for any additional elevated pressure to be formed as a result of the design of the heat accumulator as an open component.

In the case of a tall compensation line, the static pressure in the heat accumulator can reach very high values which permit “unpressurized” storage of district heat hot water up to over 135° C.

The operating pressure in the hot part of the heat accumulator (which is usually connected to the hot district heat supply) is fixed by the height of the compensation line. This operating pressure in the hot part of the heat accumulator is selected such that it comes to lie above the saturation pressure corresponding to the maximum possible district heat supply temperature (for example, in winter).

It is essential that the compensation line extends into the tank because otherwise heat would be transferred from hot water at, for example, 135° C. in the upper part of the tank to the water in the compensation line. Evaporation would consequently occur in the upper part of the compensation line (geyser effect), as a result of which the static pressure in the compensation line would fall because the density of steam bubbles is less than the liquid water. The hot water at 135° C. in the upper part of the tank would also evaporate.

The compensation line can extend into the tank as far as a lower region of the tank. Water from the upper, hottest part of the tank or also from a central part is thus not able to pass into the compensation line, and there is no need for cooling of the compensation line or alternatively intensive rinsing of the compensation line with relatively cold water.

So that the transfer of heat from the hot fluid in the upper region of the tank to the cold fluid in that part of the compensation line which extends into the tank is as low as possible, that part of the compensation line which extends into the tank is at least partially thermally insulated.

In order not to discard the fluid issuing from the compensation line, the heat accumulator comprises a collecting container for fluid issuing from the compensation line.

The heat accumulator moreover comprises a line, branching off from the collecting container, opening into a lower region of the tank and within which a pump is connected. The fluid caught can thus be recycled into the tank. Different charging/discharging mass flows and incidences of thermal expansion in the heat accumulator are compensated by the permanent filling/overfeeding of the compensation line.

In order to optimally charge and discharge the heat accumulator and correspondingly distribute the heat in the heat accumulator, the tank has a connection, arranged in a lower region, to a district heat return line, and a connection, arranged in an upper region, to a district heat supply line.

The fluid is conditioned demineralized water as water is a good, non-toxic, and readily available heat transfer medium. In order to transfer the heat between the heat accumulator and the district heat grid, there is no need for any additional heat exchangers, as a result of which there are fewer components.

There is furthermore no exergy loss owing to the terminal temperature difference in the heat exchanger during the charging and discharging of the heat accumulator. The usable heat capacity of the heat accumulator is moreover increased (the heat accumulator can be charged up to a district heat supply temperature) and finally the district heat grid can be supplied at all times from the heat accumulator at the previously stored supply temperature—without any reheating (both during charging and discharging, with heat exchangers there is a terminal temperature difference and the discharge temperature is thus always lower than the desired supply temperature by twice the terminal temperature difference).

In the method according to the invention for storing and providing heat energy, which occurs during the generation of electricity, by means of a fluid, wherein the fluid is used in a district heating grid and is stored in a tank of the heat accumulator, an operating pressure in a hottest part of the heat accumulator comes to lie above a saturation pressure of the fluid corresponding to a maximum possible district heat supply temperature by the fluid being stored in a tank, and the static pressure in the tank being increased by a compensation line which starts from the tank and extends above the latter, wherein the compensation line extends into the tank, wherein a connection of the tank interior to the atmosphere is established with an open end, opposite the tank-side end, of the compensation line, and the fluid, which is water, below an upper region of the tank passes into the compensation line.

It is thus expedient if the compensation line is kept filled so that a sufficiently high geodetic water pressure prevails in the tank at all times.

In the case of the heat accumulator, excess district heat can be stored at the usual supply temperatures of up to 135° C. (and higher, depending on the height and design of the heat accumulator) and removed when needed and fed back into the district heat grid without further reheating. At times of low grid charges, the CHP power plant can thus be completely shut down and the district heat grid can be served from the heat accumulator at the required supply temperature.

The tank of the heat accumulator can be designed as an unpressurized tank open to the atmosphere and hence is not subject to regulations on pressurized equipment. There is no need for recurring pressure and visual tests. Insurance premiums are considerably lower compared with a pressurized container.

The accumulator volumes required can be drastically reduced because it is possible for the heat accumulator, unlike pressurized tanks to date, not to be charged and discharged usually between a return temperature of approximately 60° C. and a maximum of 95° C. (i.e. with a temperature difference of approximately 35K), and instead with a temperature difference of up to 75K (for example, from 60° C. to 135° C.).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in detail by way of example and with reference to the drawings, in which, schematically and not to scale:

FIG. 1 shows a heat accumulator.

DETAILED DESCRIPTION OF INVENTION

The heat accumulator 1 in FIG. 1 comprises a tank 2 with a height of typically between 10 and 50 meters for a fluid, and a compensation line 3 which starts from the tank 2 and extends, for example, approximately 25 meters above the tank 2 in order to increase the static pressure in the latter. That end 5 of the compensation line 3 opposite the tank-side end 4 is open, as a result of which a connection of a sealed tank interior 6 to the atmosphere 7 is established.

The static pressure in the tank 2 is increased by the thin, water-filled compensation line 3 extending above the tank 2. The height of the compensation line 3 determines the maximum elevated pressure that can be set in the upper region 14 of the tank 2 and hence the maximum charging temperature, for example 135° C. The compensation line 3 is preferably extended downward through the tank 2 and is connected to the lower colder region 8 of the tank 2.

During operation, the compensation line 3 is permanently fed (for example, overfed) and hence kept filled with water which is cold (for example, at 60° C.) compared to the maximum charging temperature. The excess water flows at the open end 5 out of the compensation line 3 and falls into a collecting container 10, open to the atmosphere and situated underneath, from where it is pumped back again into the tank 2 via a line 11 within which a pump 12 is connected.

Different charging/discharging mass flows and incidences of thermal expansion in the heat accumulator are compensated by the permanent filling/overfeeding of the compensation line 3.

So that the transfer of heat in the tank 2 from the hot to the cold medium is as low as possible, that part 9 of the compensation line 3 which extends into the tank 2 is at least partially thermally insulated.

In the exemplary embodiment, the tank 2 has a connection 13, arranged in the lower region 8, to the district heat return line, and a connection 15, arranged in an upper region 14, to the district heat supply line.

The heat accumulator 1 can also be charged and discharged via these connections 13, 15. 

1-10. (canceled)
 11. A method for storing and providing heat energy, which occurs during the generation of electricity, by means of a fluid, wherein the fluid is used in a district heating grid and is stored in a tank of the heat accumulator, wherein an operating pressure in a the hottest part of a heat accumulator comes to lie above a saturation pressure of the fluid corresponding to a maximum possible district heat supply temperature, the method comprising: storing the fluid in a tank, and increasing the static pressure in the tank by a compensation line which starts from the tank and extends above the latter, wherein the compensation line extends into the tank, and wherein a connection of the tank interior to the atmosphere is established with an open end, opposite the tank-side end, of the compensation line, and the fluid, which is water, below an upper region of the tank passes into the compensation line.
 12. The method as claimed in claim 11, wherein the compensation line is kept filled. 